1. Field of the Invention
The invention relates to analyzing over time the performance of a control rod drive mechanism of a nuclear reactor, and its associated control system, by digitizing, storing and analyzing current level signals developed during stepwise lifting and lowering of control rods using electromagnetically driven sequential gripping and moving devices.
2. Prior Art
In a nuclear reactor for power generation, such as a pressurized water reactor, heat is generated by fission of a nuclear fuel such as enriched uranium, and transferred into a coolant flowing through a reactor core. The core contains elongated nuclear fuel rods mounted in proximity with one another on a fuel assembly structure through and over which the coolant flows. The fuel rods are spaced from one another in coextensive parallel arrays. Some of the neutrons and other atomic particles released during nuclear decay of fuel atoms in a given fuel rod pass through the spaces between fuel rods and impinge on the fissile material in an adjacent fuel rod, contributing to the nuclear reaction and to the heat generated by the core.
Movable control rods are dispersed throughout the nuclear core to enable control of the overall rate of fission, by absorbing a portion of the neutrons passing between fuel rods, which otherwise would contribute to the fission reaction. The control rods generally comprise elongated rods of neutron absorbing material and fit into longitudinal openings or guide thimbles in the fuel assemblies running parallel to and between the fuel rods. Inserting a control rod further into the core causes more neutrons to be absorbed without contributing to fission in an adjacent fuel rod; and retracting the control rod reduces the extent of neutron absorption and increases the rate of the nuclear reaction and the power output of the core.
The control rods are supported in cluster assemblies that are movable to advance or retract a group of control rods relative to the core. For this purpose, control rod drive mechanisms are provided, typically as part of an upper internals arrangement located within the nuclear reactor vessel above the nuclear core. The reactor vessel is typically pressurized to a high internal pressure, and the control rod drive mechanisms are housed in pressure housings that are tubular extensions of the reactor pressure vessel.
One type of mechanism for positioning a control rod is a so-called magnetic jack, operable to move the control rod by an incremental distance into or out of the core in discrete steps. The control rod drive mechanism has three electromagnetic coils and armatures or plungers that are operated in a coordinated manner to raise and lower a drive rod shaft and a control rod cluster assembly coupled to the shaft. The three coils are mounted around and outside the pressure housing. Two of the three coils operate grippers that when powered by the coils engage with the drive rod shaft, one of the grippers being axially stationary and the other axially movable.
The drive rod shaft has axially spaced circumferential grooves that are clasped by grip latches on the grippers, spaced circumferentially around the drive rod shaft. The third coil actuates a lift plunger coupled between the movable gripper and a fixed point. If control power to the control rod drive mechanism is lost, the two grippers both release and the control rods drop by gravity into their maximum nuclear flux damping position. So long as control power remains activated, at least one of the stationary gripper and the movable gripper holds the drive rod shaft at all times.
The three coils are operated in a timed and coordinated manner alternately to hold and to move the drive shaft. The sequence of gripping actions and movements is different depending on whether the stepwise movement is a retraction or an advance. The stationary gripper and the movable gripper operate substantially alternately, although during the sequence of movements both grippers engage the drive shaft during a change from holding stationary to movement for advance or retraction. The stationary gripper can hold the drive shaft while the movable gripper is moved to a new position of engagement, for lowering (advancing) the drive shaft and the control rods. The movable gripper engages with the drive shaft when moving it up or down as controlled by the lift plunger. After the movable gripper engages the drive shaft, the stationary gripper is released and then the plunger is activated or deactivated to effect movement in one direction or the other. Typically, each jacking or stepping movement moves the drive rod shaft 5/8 inch (1.6 cm), and some 228 steps are taken at about 0.8 seconds per step, to move a control rod cluster over its full span of positions between the bottom and top of the fuel assembly.
More particularly, for lifting (retracting) the control rods, the following steps are accomplished in sequence, beginning with the stationary gripper engaged in a drive rod groove and the movable gripper and plunger both being deactivated:
1. the movable gripper is energized and engages a drive rod groove; PA1 2. the stationary gripper is de-energized and disengages from the drive rod; PA1 3. the lift coil is energized and electromagnetically lifts the movable gripper and the drive rod an elevation equal to the span of the lift plunger; PA1 4. the stationary gripper is energized, re-engages and holds the drive rod (i.e., both grippers are engaged); PA1 5. the movable coil is de-energized and disengages the drive rod; PA1 6. the lift coil is de-energized, dropping the movable coil back to its start position, namely one step lower on the lifted drive rod. PA1 1. the lift coil is energized, moving the movable gripper one step up along the drive rod; PA1 2. the movable gripper coil is energized and the movable gripper grips the drive rod; PA1 3. the stationary coil is de-energized, releasing the drive rod; PA1 4. the lift coil is de-energized, dropping the movable coil and the drive rod by one step; PA1 5. the stationary coil is energized and the stationary gripper engages the drive rod, at a position one step higher than its previous position; and, PA1 6. the movable coil is de-energized and the movable gripper disengages from the drive rod.
Similarly, for lowering (advancing) the control rods, the following steps are accomplished in sequence, again beginning with only the stationary gripper energized:
A number of particular coil mechanisms and gripper mechanisms are possible. Examples of coil jacking mechanisms with a stationary gripper, a movable gripper and a lifting coil as described are disclosed, for example, in U.S. Pat. No. 5,307,384--King et al., U.S. Pat. No. 5,066,451--Tessaro and U.S. Pat. No. 5,009,834--Tessaro, all of which are hereby incorporated.
Whatever mechanical arrangement is employed for the grippers and lifting coil/armature arrangement, a discrete time interval is needed to complete each sequential operation. In order to move the control rods quickly, reliably and efficiently, the respective grippers and coils must be operated accurately as to their timing. This requires that the coil energizing electric power signals to the respective coils be accurately timed.
The power level of coil energization can be simply on and off, or preferably the coils can be energized at different levels during different operations in the sequence. For example, the lift coil signal can have an intermediate or "hold" current level for the lift coil, between the de-energized (zero) level and the full-on lifting level. At the intermediate level, the lift coil maintains the position of the movable coil. An intermediate current level may also provide sufficient power to move the movable gripper when it is disengaged from the control drive rod. An intermediate current level can be used with the stationary gripper coil for holding, as opposed to a higher level for positive initial gripping.
The coil signals are switched between the levels in a coordinated manner by a logic controller. The logic controller generates timed signals to switch power regulation circuits on and off or between current levels. The timing relationships between and among current pulses applied to the stationary gripper coil, the movable gripper coil and the lift coil, are adjusted manually when setting up the control rod drive mechanism, and remain set. For example, an oscilloscope or chart recorder is set up to record the three coil currents. A timing signal at a known frequency can be provided and recorded together with the coil current signals as a reference. The actual determination of timing is done by reviewing the recorded signals and spacing the operations in time sufficiently to complete each step in the sequence before undertaking the next.
The power regulation circuits attempt to maintain the required coil currents at the required times in the sequence of operations. However, the actual coil currents have current variations that reflect the status of the electromechanical operation. For example, the drive currents to the gripper coils typically show a brief reduction or notch in current when the associated gripper fully pulls in and the inductance of the gripper coil increases. The notch disappears as the power regulator responds to bring the current level to nominal. By noting the occurrence of the notch in the gripper coil current signal, the technician can determine the point in time at which the gripper engaged. The timing of the next operation, which depends on successful gripper engagement, is then set to occur at a slight delay after the gripper engagement time noted.
Various circuit elements can be used in the control logic circuits to trigger the power regulator to generate the necessary control currents at preset times in the sequence. For example, cycles at a known frequency (such as the recorded timing reference signal) can be digitally counted, and a particular power regulation switching operation can be triggered by a signal gated from a cycle counter at a predetermined count. Alternatively, the timing can be set using other means such as adjusting the RC delays of monostable multivibrators that trigger one another in a cascade sequence. In any event, the timing is predetermined and set, giving some leeway or extra time to ensure completion of each operation before commencing the next, while nevertheless enabling the rods to be advanced or retracted relatively promptly through their span of movement.
It would be advantageous to provide a more automated control mechanism that is responsive to operation of the device rather than preset timings based on the expectation that current levels will be reached and mechanical actions will occur at the needed times. It would also be advantageous if the control mechanism could include ongoing diagnostic and feedback capabilities, such that deviation from expected operation can be detected by comparison with historical or nominal operation and suitable corrective action taken.